Problem: Multiply.
$46 \times 39$ and $4.6\times 3.9$ multiply the same digits in the same order. So, the product of both problems will also have the same digits in the same order. Let's multiply $46 \times 39$. Then we can estimate to place the decimal point in the product of $4.6 \times 3.9$. $\begin{aligned} 46&\\ \underline{ \times 39}&\\ 54}& {9} \times{6\text{ ones}}\\ 360}& {9} \times{4\text{ tens}}\\ 180}& {30} \times {6\text{ ones}}\\ \underline{+1{,}200}}& {30} \times {4\text { tens}}\\ 1{,}794 \end{aligned}$ $46 \times 39 = 1{,}794$ Let's estimate to place the decimal in $4.6\times 3.9$. $\begin{aligned} 4.6 \times 3.9 &\approx 5\times 4\\\\ &\approx 20 \end{aligned}$ Where can we place the decimal in $1{,}794$ to get a product close to $20$ ? $17.94 = 4.6 \times 3.9$